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Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging



On Wed, 4 Jun 2008, Steeve McCauley wrote:

Is this change meant to add cluster support for merging?  I had
been under the assumption that merging was already a feature of
snapshots.

My implementation has nothing to do with clustering.

I think there were some userspace programs for off-line merging of inactive snapshots. My implementation works in kernel and can be used while the snapshot is mounted.

What is the difference between the current implementation using
lvremove (which I assumed was where the COW blocks were merged back
into the original volume) and lvconvert as below?  Why was this
functionality not kept in lvremove?

lvremove drops the snapshot and forgets its content. lvconvert -M copies the snapshot to the origin (I'm now working on to let it automatically drop the snapshot when the merging finishes).

Mikulas

Mikulas Patocka wrote:
Hi

Here I release the first experimental implementation of snapshot merging. Merging allows you to copy data in snapshot back to the origin device. Once merging starts, it runs on background. When the merging finishes (you see "0%" with "lvs" command), you should remove the merging snapshot with lvremove command.

The userspace will still be changed, the kernel is already finished (there are no known bugs in the kernel, I'm curious about the unknown ones :)

Merging is initiated with "lvconvert -M vg/lv_snapshot" command. When you initiate merging, make sure that the origin device is not mounted. Snapshot can be mounted while it's merging is initiated.

During merging, aby reads and writes to the origin device are identical to accesses to the merging snapshots.

There may be multiple snapshots while one of them is being merged --- exceptions in other snapshots are being allocated and there snapshots are kept stable.

Don't try to concurrently merge more than one snapshot (the kernel refuses to do it, the userspace tools don't know about it --- the userspace will be rewritten anyway).

The patches are at:
http://people.redhat.com/mpatocka/patches/
The patches are for 2.6.26-rc4

(besides merging, there are some other bugfixes, I made for the kernel)

Mikulas

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--
Steeve McCauley
Director of Research and Development
Rackable Systems
407 McGill Street, Suite 1002
Montreal, Quebec, H2Y 2G3 (Canada)
Tel: 514-288-3343
http://www.rackable.com
:wq

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